AD Plant Smell

The AD plant at Warminster, which runs on food waste, is pretty smelly at times, despite concerns about a possible smell nuisance being expressed at the planning stage. I certainly wouldn't want to live in any of the houses that are within 300 metres of the site.

We all know that a vacuum tanker produces a smell from the expelled air when loading. In the case of an AD plant, I would have thought that it would be perfectly possible to return the exausted air back to the digester through an additional hose. Discuss.

Edit. This thread was supposed to be in the Renewable Energy catagory.
 
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Dan7626

Member
We have a huge AD plant across the field from us and it stinks. They take the majority of food waste from across this part of Gloucestershire. They have 4 huge lagoons which they are constantly tanking out of or pumping out of across the fields.
None of the local village have a good word to say about the place. Not just because of the smell but because of the increase of traffic through the village.
 

sjt01

Member
Mixed Farmer
Location
North Norfolk
None of the food waste digesters I have visited have had any smell outside the reception hall. That is where the smell will predominate and if it is fitted with double airlock doors, and extraction through filters there is no external smell.
Tanker exhaust could be routed through such a filter. If it smells, the management could be improved!
 

Boysground

Member
Mixed Farmer
Location
Wiltshire
I know the Warminster plant have put a lot of effort into keeping the smell down. There was a lot of opposition from the locals when it went through planning. Went past the codford operation spreading digestate the other day and that was quite pungent.

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Boysground

Member
Mixed Farmer
Location
Wiltshire
It's actually at the old Chitterne dairy. If you drive Heytesbury to Chitterne at night the glow to your right is the plant. Lagoons also at Stockton I believe.

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thesilentone

Member
Livestock Farmer
Location
Cumbria
If you are handling anything that is ' waste ' there is smell, AD Plant or not, please accept we cannot live in a smell-free world.
However these tend to be confined to rural areas away from people density, and anyone who lives in an area where there are dairy cows, live with much worse, believe me.
But, an AD Plant that is processing food waste, is under strict regulation regarding smell, controlled by Odour Units. Therefore, as part of the planning regulations, the plant must comply with these unit values and in the majority of cases, there is little to no odour.
Fresh waste has low odour, older de-composing material smells, so plants are regulated to process material within as given time window.
There are also strict regulations regarding the storage of waste and odour control.
The waste must be stored inside a building that is under negative pressure, the exhaust (air) from this building must pass into a bio-filter that is fit for purpose (to get to the regulated odour unit value for the plant as agreed in the odour management plan).
When a vehicle carrying waste arrives at the plant, the door to the building must open, the vehicle enters and the door closes. The vehicle tips it's load, carry's out a bio-security clean, then the door re-opens and vehicle leaves and the door closes behind him.
If your local plant smells, someone is not following good practice, and you should rightly complain - TO THE PLANT OWNER FIRST !!!!
However, this is a difficult one, as country dwellers and city dwellers have different views on smell and odour....

In response to the comment regarding the vent on a vacuum tanker. De-gassed digestate (the by-product) has very little odour as the volatile solids and proteins have been decomposed as part of the process and turned into biogas.
 
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When I and other local farmers had a tour of the Warminster AD plant, just after it opened, I think that we were told that it is a continous process. New material is put in all the time and digestate is removed in a similar manner and taken away for spreading on land. Presumably it's a matter of a gallon removed for every one put in. Theoretically some material never comes out, and some comes out almost straight away. On that basis the output material is bound to stink.
Having had food waste, in the form of waste from chicken factories, abotoirs and breweries, injected onto our stubbles, I know all about the smell. It was done during a hot August and I was not popular. I was purseuded to use it by my agronomist, but I never did it again.
 

thesilentone

Member
Livestock Farmer
Location
Cumbria
Theoretically some material never comes out, and some comes out almost straight away. On that basis the output material is bound to stink.

You make a very good point, this material is called 'wash through' and represents max 1.5% of the total volume.
However the system is designed to deal with this, as well as the primary tank where most of the digestion takes place (over 90%) the material then enters a secondary digester where any remaining VS is degraded before entering the final storage tank/lagoon.

In the case of food-waste plants, the process can also perform waste recovery (waste in a the front end - product out at the back end). To achieve this, the process plant must comply with QSI PAS110 see: http://www.wrap.org.uk/content/bsi-pas-110-producing-quality-anaerobic-digestate

This standard sets a number of limits, one of which being the RBP (residual biogas potential). This is the amount of VS left un-degraded by the process. This low limit is tough to achieve, however it ensures an efficient digestion process and quality end product.
 

MickMoor

Member
Location
Bonsall, UK
As I understand it, digestate is anaerobic, by definition, when it comes out of the digesters. As @thesilentone has already explained, the digestion process is not complete. When it is spread, it becomes aerobic,and this process can create a pwerful odour. While still n storage it shpuld be aerated while the noxious gases can be 'filtered' out.
A biofilter is actually no such thing. The gases are passed OVER something with a large surface area, in the hope that bacteria present on this surface will capture the smelly bits.
 

thesilentone

Member
Livestock Farmer
Location
Cumbria
As I understand it, digestate is anaerobic, by definition, when it comes out of the digesters. As @thesilentone has already explained, the digestion process is not complete. When it is spread, it becomes aerobic,and this process can create a pwerful odour. While still n storage it shpuld be aerated while the noxious gases can be 'filtered' out.
A biofilter is actually no such thing.The gases are passed OVER something with a large surface area, in the hope that bacteria present on this surface will capture the smelly bits.

A couple of points, after a two stage digestion process, very little VS is left, PAS110 RBP test ensures almost 100% breakdown.
There should be no need to ' aerate ' in a storage tank, there is a need to agitate, however this is just to avoid separation and settlement.
At a waste plant all the filtration and capture of odorous air and gas cleaning is carried out long before final storage.
The final storage tank is usually not heated, so a low settlement temperature should occur, therefore no further fermentation.
I don't understand what you mean with your comment : A biofilter is actually no such thing
At a waste plant, we have to deal with odour, that is a planning condition.
Cleaning gas is different, this can be done in several ways, one of which may be a biofiler (biological filter)
However, the same technical name is given to a filter for cleaning air - biofilter, these again come in many types depending on the levels of cleaning required.
Regarding your final comment, simple gas cleaning is usually inside a digestion tank where a combination of H2S eating bacteria (grown on a floating layer or slats, or nets etc) are only the first stage, this process includes the addition of oxygen (you term as aerate) after this the gas is still not clean enough to burn in a CHP, therefore some form of secondary cleaning is required. This secondary filter could be biological or active carbon etc.............

 

MickMoor

Member
Location
Bonsall, UK
A
A biofilter is actually no such thing

In the eyes of Joe public, if he or she thinks back to their science lessons, things pass through a filter, and it retains thigs that will not. A biofilter , as such, relies on gasses passing over it, it will never remove all of the impurities. It is some time since I did any work on odour removal, but according to the chap I worked with, who now lectures in the subject at Dublin University, it is just a matter of reducing odour to an acceptable level.
 

thesilentone

Member
Livestock Farmer
Location
Cumbria
I think I understand your comment, however for secondary cleaning I refer to gas cleaners where the gas passes through not over, and where a biological process takes place........
 

Wastexprt

Member
BASIS
De-gassed digestate (the by-product) has very little odour as the volatile solids and proteins have been decomposed as part of the process and turned into biogas.

They were spreading AD digestate (food based and pig slurry) from our local AD plant last week, it absolutely stank. Most of the whiff went when they worked it in, still slightly aromatic though.

I've heard a few accounts of H2S alarms going off in fields whilst AD digestate spreading.....
 

MickMoor

Member
Location
Bonsall, UK
They were spreading AD digestate (food based and pig slurry) from our local AD plant last week, it absolutely stank. Most of the whiff went when they worked it in, still slightly aromatic though.

I've heard a few accounts of H2S alarms going off in fields whilst AD digestate spreading.....

Despite earlier posts, I still maintain that when spreading digestate, you are turning a liquid that was anaerobic into an aerobic one, hence the offensive odour.The answer is to aerate while the digestate is in storage. I installed a system at site in N. Wales a few years ago, and this was how it was explained to me. I have not heard as to how effective it is, but as they say,no news is goood news.
 

Wastexprt

Member
BASIS
Despite earlier posts, I still maintain that when spreading digestate, you are turning a liquid that was anaerobic into an aerobic one, hence the offensive odour.The answer is to aerate while the digestate is in storage. I installed a system at site in N. Wales a few years ago, and this was how it was explained to me. I have not heard as to how effective it is, but as they say,no news is goood news.

I have a client who wants to aerobically treat liquids, the EA were quite keen on them accepting AD digestate as, in their words, it would lead to a 'significant odour reduction'.
 

thesilentone

Member
Livestock Farmer
Location
Cumbria
No one disputes your comments regarding the environment between anaerobic and aerobic, however this thread was started regarding the odour around a plant and what regulations are in place to prevent this.

Once the anaerobic material enters an aerobic environment there is some odour as I clearly state in my first response.
When compared, de-gassed material has less odour than fresh (aerobic de-composed) material as the proteins have been removed en-route, but there is still ammonium and sulphur (ammonium from the nitrogen and sulphur as an element) which has a odour. The dispersal rate of this odour compared to ' fresh ' material is much faster, as is the uptake of the nutrients.
If you 'aerate' this in a liquid form, you require a vent or open tank, depending on ambient temperature, you now run the risk of evaporation of the N to atmosphere, which rather defeats the objective.
 

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